The present invention relates to a camshaft drive system for operating an internal combustion engine according to the two-stroke cycle or the four-stroke cycle.
Conventional internal combustion engines operate according to thermodynamic principles following either the two-stroke cycle or the four-stroke cycle and are commonly classified as two-stroke engines or four-stroke engines respectively. Both type of engines can operate using a range of fuels including gasoline, diesel, alcohol and gaseous fuels; the fuel can be introduced into the engine by a range of devices including carburettors and fuel injectors into the air intake system and fuel injectors aimed directly into the engine cylinder; and the fuel-air mixture can be ignited by a range of methods including spark ignition and compression ignition. Each engine cycle type has different merits and shortcomings making it a subject of continuous debate as to which is more suitable for automotive application taking into account various parameters such as power density, fuel consumption, exhaust emissions, noise, vibration, engine size, weight and cost etc. Currently, the automotive manufacturer must first decide on the engine cycle type, then design the engine accordingly.
With a view of combining the merits of both engine cycle types, there is provided a camshaft drive system for a selectable two-stroke/four-stroke internal combustion engine with exhaust and intake poppet valves opened and closed by at least one camshaft, wherein the camshaft is connected permanently to a planetary gear set which is driven continuously by the engine crankshaft, and wherein means are provided for setting the planetary gear set in one of two selectable stable modes to function either as a gear box or as a rigid coupling.
In a preferred embodiment of the invention, the camshaft is driven by the planetary gear set according to the selectable mode at either the same speed as the engine crankshaft or half the speed of the engine crankshaft, and the phasing of the camshaft in relation with the crank angle position of the piston is adjustable using the planetary gear set to position the exhaust and intake gas exchange events appropriately for either the two-stroke or the four-stroke engine cycle respectively.
Preferably, the planetary gear set is mounted concentrically to the camshaft. Alternatively, it may be mounted concentrically to the engine crankshaft or it may be mounted separately next to the camshaft and the crankshaft.
Various configurations are possible in using the planetary gear set to drive the camshaft according to the present invention. The input from the engine may be connected to any one of the three members of the planetary gear set which may be the ring gear, the sun gear or the planetary gear carrier, the output to the camshaft may be connected to any one of the remaining two members, and the mode setting means may be provided in the remaining last member which is the reaction member between the input and output members.
In the case where the planetary gear carrier is the reaction member, idler gears coupled with the planetary gears may be additionally provided on the planetary gear carrier such that the ring gear and the sun gear rotate in the same direction.
In the invention, the means for setting the planetary gear set to function as a gear box is a device for holding the reaction member of the gear set stationary relative to a fixed datum so that the gears rotate relative to one another within the gear set, whereas the means for setting the planetary gear set to function as a rigid coupling is a device for holding the reaction member of the gear set stationary relative to either the rotating input member or the rotating output member so that the gears are locked relative to one another within the gear set and the whole assembly rotate together.
In a preferred example, the ring gear is the input member driven by the engine at half crankshaft speed, the sun gear is the output member connected to the camshaft, and the planetary gear carrier is the reaction member of the gear set. The speed ratio between the ring gear and the sun gear is 1:2.
In this example, means are provided for setting the planetary gear carrier in one of two selectable stable modes such that in the two-stroke mode the carrier is held stationary relative to the engine frame so that the gear set acts as a gear box, and in the four-stroke mode the carrier is held stationary relative to the rotating ring gear so that the entire gear set acts as a rigid coupling. Additionally, means are provided for allowing the carrier to slip and rotate relative to the engine frame or relative to the rotating ring gear in order to adjust the phasing and the speed of the camshaft during a mode change.
In operation, when the two-stroke mode is selected, the carrier, which has been held stationary relative to the rotating ring gear in the four-stroke mode, is first allowed to slip and rotate relative to the rotating ring gear in order to adjust the phasing and the speed of the camshaft progressively towards the two-stroke mode, at which point the carrier is then held stationary relative to the engine frame thereafter. In this case, the planetary gear set acts as a gear box in which the camshaft is driven at twice the speed of the ring gear and each engine valve along the camshaft is opened and closed at a predetermined two-stroke valve timing once during every engine revolution consistent with the two-stroke cycle.
In addition to the above, a combination of various operating parameters in the engine necessary for executing the two-stroke cycle is implemented, and this would be obvious to a person familiar with the state-of-the-art. For example, an air blower would be necessary to supply pressurised air for the two-stroke scavenging process and the frequency and timing of the fuel injection and/or spark ignition would be scheduled appropriately.
When the four-stroke mode is selected, the carrier, which has been held stationary relative to the engine frame in the two-stroke mode, is first allowed to slip and rotate relative to the engine frame in order to adjust the phasing and the speed of the camshaft progressively towards the four-stroke mode, at which point the carrier is then held stationary relative to the rotating ring gear thereafter. In this case, the planetary gear set acts as a rigid coupling in which the camshaft is driven at the same speed as the ring gear and each engine valve along the camshaft is opened and closed at a predetermined four-stroke valve timing once during every two engine revolutions consistent with the four-stroke cycle.
In addition to the above, a combination of various operating parameters in the engine necessary for executing the four-stroke cycle is implemented, and this would be obvious to a person familiar with the state-of-the-art. The air blower for two-stroke operation may in this case be used to supply boosted air for four-stroke operation.
The above example is only one of many possible configurations for the camshaft to be driven by a mode selectable planetary gear set of the present invention. In another example, the planetary gear carrier is the input member driven at half crankshaft speed, the sun gear is the output member connected to the camshaft, and the ring gear is the reaction member of the gear set. The speed ratio between the carrier and the sun gear is 1:2.
In this case, the ring gear may be set in one of two selectable modes such that in the two-stroke mode the ring gear is held stationary relative to the engine frame so that the planetary gear set acts as a gear box and the camshaft is driven at crankshaft speed, and in the four-stroke mode the ring gear is held stationary relative to the carrier so that the entire gear set acts as a rigid coupling and the camshaft is driven at half crankshaft speed.
In a further example, the sun gear is the input member driven at crankshaft speed, the planetary gear carrier is the output member connected to the camshaft, and the ring gear is the reaction member of the gear set. The speed ratio between the sun gear and the carrier is 2:1.
In this case, the ring gear may be set in one of two selectable modes such that in the two-stroke mode the ring gear is held stationary relative to the carrier so that the entire gear set acts as a rigid coupling and the camshaft is driven at crankshaft speed, and in the four-stroke mode the ring gear is held stationary relative to the engine frame so that the epicycle gear set acts as a gear box and the camshaft is driven at half crankshaft speed.
The principle of operating a planetary gear set in several selectable modes is well known and is commonly used in automotive automatic transmission systems for driving a vehicle. The present invention proposes to apply the same principle for driving a camshaft in order to enable the engine to be operated in either the two-stroke or the four-stroke mode with smooth and rapid transition and with the minimum risk of misfire during a mode change even at high speed. This provides a camshaft drive which is permanently in mesh and avoids the abruptness and uncertainty caused by temporarily decoupling and then re-coupling the camshaft and the crankshaft as proposed in the previously known systems such as U.S. Pat. No. 4,907,544 and U.S. Pat. No. 2,178,152 which are unacceptable for automotive application.
The technology for controlling the planetary gear set to operate in the selectable modes and to change mode smoothly and imperceptibly using brakes, clutches, and sensors is well advanced in the automatic transmission field and may be transferred readily to the camshaft drive system of the present invention.